FIG. 1. shows a schematic cross section of a prior art headlamp, showing a magnified section of a reflective wall portion. The headlamp 10 includes a plastic resin wall 12 that commonly includes fill materials. Over the wall 12 is placed a base coat 14 that smoothes out and seals over the irregularities of the plastic resin wall 12. The smooth base coat 14 is then metallized with a reflective layer 16 (aluminum). The reflective layer 16 is further coated with a clear protective coating 18 to prevent tarnishing or other injury to the reflective layer 16.
Headlamps are being sculpted to conform with the aerodynamic shapes of vehicles. It is convenient to use molded plastic resin for the headlamp, including the reflector portion directing the light onto the road. To be effective, headlamps require fine optical reflectors to accurately and efficiently reflect the available light. It is known that high quality reflectors can be made from pure resins, particularly, high cost resins such as polycarbonates, polyether-imide, polyphthalate-carbonate, polyarylsulone, and other engineering thermoplastics. To avoid the high cost material, headlamps are commonly made from a combination of a moderately priced resin, glass fiber and an inexpensive filler such as talc, calcium carbonate, or others. The glass fiber improves the strength and the filler lowers the overall cost of the reflector, and improves dimensional stability. Unfortunately the fiber and fillers can appear at the surface, causing a rough, pitted or otherwise irregular surface. If the rough surface is metallized directly, the resulting reflector can be hazy, pitted, and otherwise irregular. A substantial portion of the reflected light is then lost, or misdirected.
This problem is so important that, a base coat is commonly applied over the region where the reflector is to be formed. The base coat evens the rough surface to provide the smooth base needed for a fine reflector. Unfortunately, base coating materials are expensive, and environmentally noxious. The base coats can also run, drip, or build up in irregular ways in seams, or crevices. These irregularities can cause a headlamp to be rejected for cosmetic reasons, or because of the irregularly directed light from these regions causes too much glare. The base coat must therefore be applied with great care, usually meaning, slowly in controlled patterns and then cured in a controlled environment. Applying the base coat is then expensive, noxious, and time consuming. There has been a need for a glass fiber and mineral filled headlamp that has a fine reflector surface that does not need a base coat. This need has been recognized for some time, and there have been several attempts to satisfy this need. None of these attempts have been sufficiently successful to provide a quality headlamp reflector. There is then a long recognized need for a glass fiber and mineral filled headlamp that requires no base coat.
In the past it was known to make molded headlamp reflectors by injection molding, and injection compression molding. Bulk material is fed from a hopper by an auger into a heated barrel. Once in the injection barrel, the bulk material is warmed by the heat from the barrel and by the shearing force of the screw moving the material forward, as a load or shot fills the mold. The screw feed also causes the bulk material to become warm, and plastic. After being warmed, the shot is forced under pressure from the injection screw into a heated mold, where it ideally fills all the voids in the open volume of the mold. The pressure of the feed material forces the warm plastic into the mold where it should fill all of the open volume, and then cure sufficiently to be released as a firm piece. The material under heat and pressure then is cured sufficiently to be released as a firm piece. The same process occurs in the injection compression process, except that a further compression is added to the material while it resides in the mold. Standard ranges of parameters for these methods are set out in the following table:
TABLE 1 ______________________________________ Prior Art Injection- Molding Parameter Injection Compression ______________________________________ Barrel Temperature 80 to 110 80 to 110 (.degree.F.) Back Pressure (psi) 20 to 50 20 to 50 Barrel Residence not applicable not applicable Time (min) can be hours can be hours Compression Gap not applicable 0.06 to 0.09 (inches) Injection Speed 2 to 5 2 to 5 (seconds) Injection Flow Rate &lt;300 &lt;300 (cc/second) Injection Pressure &lt;1500 &lt;1500 (psi) Cushion: 0.05 0.05 Hold Pressure (psi) 300 not applicable Hold Pressure Time 3 to 10 not applicable (seconds) Mold Surface 280 to 340 280 to 340 Temperature (.degree.F.) Cure Time (seconds) &gt;40 &gt;40 ______________________________________
The Applicant in attempting to satisfy production needs for a no base coat headlamp asked five suppliers for their best samples. These samples were tested and found to have surface glosses that varied from 2 to 75 using ASTM test E430 at 20.degree.. It is estimated that a value of 80 or more is necessary to provide a vehicle headlamp that is sufficiently efficient, and has an acceptably low glare. There is then a need for a method of making a glass and mineral filled vehicle headlamp with a surface gloss greater than 80.
In the present invention, the molding process has been improved to enable the production of parts that meet the above standards. The development of the relationship between the molding parameters that is described and the resulting surface quality of the molded article capable of meeting the above requirements, is believed to be novel. The lamp reflector with no base coat is also believed to be novel.